1. The Field of the Invention
The present invention is directed to semiconductor device assemblies. More specifically, the present invention is directed to a method for forming connections, such as ball grid arrays, on a semiconductor chip, substrate or semiconductor package.
2. Background of the Invention
As integrated circuits become larger and more complex, the demand for the increased number of external connections on chips, semiconductor substrates and packages has rapidly grown. The most common methods for connection of the packaged devices to external systems are wire bonding, tape automated bonding and solder bumping. Among these technologies, solder bumping provides the highest packaging density with less packaging delay. Therefore, solder bumping or creation of ball or bumped grid arrays (BGAs) is rapidly developing as the technology of choice for high input/output count integrated circuits. A BGA design is part of an integrated circuit package in which the input and output points are solder bumps (or balls) arranged in a grid pattern. BGA designs are used in various package configurations, including a single chip or multichip applications.
Solders have been commonly used as interconnecting material for electronic packaging and assembly. A typical ball grid array package has a large number of solder balls or bumps disposed on a surface of the package. The package surface is usually formed from an electrically insulating material with a pattern of metalized pads disposed on it. These pads connect to circuity of a semiconductor device within the package. BGA packaging technology offers superior thermal, electrical, and size performance compared to other existing package technologies. One challenge affecting BGA technology, however, is to achieve the interconnect density required to route area array devices or packages, at a cost compatible with the commercial marketplace. Another concern is uniformity of the balls across a semiconductor device during fabrication of solder ball arrays. Finally, a badly bonded ball or voids in the solder balls can lead to faulty operation of the device.
Several different methods have been developed for forming ball grid arrays or solder bumps. Many of these methods use preformed solder balls and place these preformed balls on the package contact pads. For example, some methods use reflow soldering which is a process for joining parts to a substrate by depositing solder paste, placing parts (for example, preformed solder balls), heating until the solder fuses, and allowing them to cool in a joined position. Since it is very difficult to ensure that the correct amount of paste is applied to the electrical contacts, these methods are complicated, time consuming and not cost-effective.
As an alternative, a solder paste is disposed on the contact pads in specifically measured quantities. Thereafter the paste is heated and while it melts, it assumes the spherical shapes to form the ball grid contacts. This method also has a number of disadvantages because of the difficulties with measuring the exact quantities of solder paste and dealing with the elasticity, viscosity and other important characteristics of the paste. The smaller contact pads and narrower spacing between the contact pads require a higher level of precision in solder paste deposition and in component placement, and require higher quality and consistency in solder paste. Moreover, the shape and the size of the resulting balls are often not uniform because it is difficult to control the wettable exposed area of the contact pad covered by the solder paste while it is melting. Therefore, this and similar methods inevitably result in a complex manufacturing process with rigorous process control and the resulting higher costs.
Another conventional method of creation of the solder bumps is by evaporation. Evaporation does not create uniform solder ball heights and can be time-consuming. Similarly, wire bonding with solder has not been able to yield consistent ball size and adhesion strength.
One example of the method for forming raised bump contacts on a substrate is disclosed in the U.S. Pat. No. 5,381,848 to Trabucco which is fully incorporated herein by reference. In that patent, a substrate upon a surface of which the solder bumps are formed is inserted into a two-halves mold with a number of the specially formed recesses. The molten solder thereafter is introduced into the recesses. Upon cooling inside the recesses, molten solder solidifies and forms bumps on a substrate in the shape of the recesses of the mold. Only after solidification of the solder, the substrate is removed from the mold. Another invention that requires the molten solder to solidify before the device is removed from the mold is disclosed in the U.S. Pat. No. 5,244,143 to Ference at al. Both of the above-mentioned inventions still require strict quality and process control and inherent maintenance of the mold, including the verification of the precise formation of each recess in the mold to assure the identical shape and size of the resulting solder bumps. Any imperfections in the shape or dimensions of the recesses of the mold result in the non-uniform balls, and thus, potentially defective contacts. Moreover, some damage to the bumps may occur while opening the mold and removing the device from the mold.
Therefore, with the requirements of the increased complexity and density of the BGAs, there is an increased need for a method for forming solder bumps that is simple, cost-effective and time-effective. There is also a need for method that allows for batch processing of the highly uniform solder balls that requires relatively low investment in manpower and complicated equipment. It would be also advantageous to create a method of forming solder bumps that lends itself to high-volume production, and that has a low defects rate and a higher yield.
In light of the foregoing, it would be an advantage in the art to devise a time- and cost-effective solder ball formation process that forms solder balls having substantially uniform dimension, preferably in batch quantities, on a substrate.